1. Field of the Invention
The present invention relates to image sensors configured with color filter arrays.
2. Description of the Related Art
Imaging systems, such as digital cameras, are used for still photography and video recording. The images captured by the system may be used for viewing/processing in a variety of representations, such as electronic, digital, or printed. For color imaging, image data are typically captured in three different colors, e.g., red, green, and blue. When the three sets of data representing the colors are combined, a color image of the scene is created. Capturing these three sets of data can be achieved in a number of ways. In digital imaging, this is often accomplished by using a two-dimensional sensor array comprising photosensitive pixels that are covered by a pattern of red, green, and blue filters, which pattern is known as a color filter array (CFA).
FIG. 1 shows a schematic block diagram of a digital camera 100. Camera 100 uses a lens 102 to collect light from a scene. The light is captured by a color sensor array (CSA) 104 comprising a CFA 106 and a sensor array 108. Light passes through and is filtered by CFA 106 and is converted into digital signals by sensor array 108. The resulting image data are stored in and/or processed by a memory/processor block 110. The data can then be output from camera 100 to external devices through an interface 112. Sensor array 108 may be a CCD array, a CMOS array, or some other one- or two-dimensional imaging device.
FIG. 2 shows one possible implementation of CSA 104 of FIG. 1. Photosensitive pixels with associated color filters are represented by rectangles in the grid of FIG. 2. An R indicates a pixel having a red filter. Similarly, pixels with a green or blue filter are indicated by a G or B, respectively. Color filters in CFA 106 of CSA 104 are arranged in a particular pattern, known as the Bayer pattern, as described in U.S. Pat. No. 3,971,065, the teachings of which are incorporated herein by reference. A kernel 202 of the Bayer pattern has four color filters in a 2×2 arrangement. Starting from the upper left corner and going clockwise, the color sequence within kernel 202 is green-red-green-blue (GRGB). This kernel is replicated throughout CFA 106 in both the horizontal and vertical directions.
In the example shown in FIG. 2, only a central region 204 of CSA 104 is used to generate an image. A 2×2 block 206 of color filters of CFA 106 in the upper left corner of region 204 has the desired GRGB Bayer sequence. Region 204 is surrounded by several rows/columns of extra pixels, the signals of which are not represented in the image. Having such extra pixels is common practice in image sensors. Only some of the pixels outside central region 204 have color filters. An O indicates a pixel that does not have a color filter. In the particular example of FIG. 2, the row/column at each edge of array 106 has pixels without color filters.
Fabrication of a CSA (e.g., by deposition of a CFA onto a sensor array) is a separate step in the image sensor manufacturing process. Typically, it is carried out at a separate facility after the sensor array, such as array 108 of FIG. 1, has already been fabricated. A common manufacturing defect is misalignment of the CFA relative to the sensor array. Frequently, the misalignment involves a shift by one row and/or one column between the CFA and the sensor array.
FIG. 3 shows a CSA 300, similar to CSA 104 of FIG. 2. However, in CSA 300, CFA 302 is misaligned relative to the sensor array by being shifted one column to the right. A 2×2 block 306 of color filters of CFA 302 in the upper left corner of central region 304 of CSA 300 forms an RGBG sequence as opposed to the desired GRGB sequence of the Bayer pattern. If a digital image is produced using CSA 300, severe color distortion may result.
FIGS. 4 and 5 show two other possible misalignments. In FIG. 4, CFA 402 of CSA 400 is shifted down by one row resulting in a BGRG sequence for a 2×2 block 406 of color filters of CFA 402 in the upper left corner of central region 404 of CSA 400. In FIG. 5, CFA 502 of CSA 500 is shifted by one row down and by one column right resulting in a GBGR sequence for a 2×2 block 506 of color filters of CFA 502 in the upper left corner of central region 504 of CSA 500. Similar to CSA 300, the misalignments in CSAs 400 and 500 may cause severe color distortions.